As a protective film employed in the polarizing plate of a liquid crystal display, cellulose esters such as triacetyl cellulose and the like, are suitable due to their lower double refraction, and have frequently been employed.
Commonly, a polarizing plate has such a structure that a polarizing film comprised of a polyvinyl alcohol film and the like, in which iodine or dyes are absorbed and oriented, is laminated on both sides with transparent resin layers. Frequently employed as said transparent resin layer is a protective film comprised of triacetyl cellulose film.
When a polarizing plate is produced by adhering a protective film with a polarizer, in order to readily apply a water-soluble adhesive, a protective film such as triacetyl cellulose film, and the like is temporarily subjected to saponifying treatment through immersion in an alkali solution having a high concentration at a relatively high temperature so that the film surface becomes hydrophilic. Then, an adhesive is applied to said protective film which is adhered with the polarizer. However, it is preferred to make a transparent resin film hydrophilic without employing chemicals for saponification, which are not preferred in view of work as well as troublesome processes.
Further, for a transparent resin film employed as the protective film for the polarizer, triacetyl cellulose is exclusively employed. One of the reasons why triacetyl cellulose has not been replaced by other films is that the other films are not subjected to saponification. Accordingly, another process, which replaces saponification, has been sought.
The present inventors have investigated various methods to make the surface of a transparent resin film hydrophilic, which replace saponification. As a result, it has been discovered that when a transparent resin film is subjected to plasma treatment, it exhibits similar performance obtained by saponification.